The invention relates to an input circuit for relatively high current alternating voltage signals to be monitored, which alternating voltage signals are to be monitored relative to a pre-settable amplitude threshold value. Such input circuits are employed for example in small control circuits.
Input circuits for monitoring of alternating current signals are generally known and by way of example in printed patent document FR 2671437 A1 recited here. In order to maintain a low dissipation power loss, such input circuits require small input currents. Some devices to be monitored, for example switches furnished with a glow discharge lamp or proximity switches require, however, higher input currents into the input circuit for a problem free functioning. It is usual for these cases to furnish the input circuit with a parallel connected capacitor on the input side in order to assure the necessary current for the device to be monitored. The disadvantage of this construction comprises that upon switching off of the device to be monitored in the region of the apex of its alternating current supply voltage, then the capacitor can discharge only relatively slow starting from a high-value through the high ohm input resistance. Furthermore a residual current flows through such devices in a disconnected state, wherein the residual current causes a substantial residual voltage at the input. The two processes enable the perception of the switched off state of the device to be monitored only after a plurality of alternating voltage periods of the supply voltage and notxe2x80x94as desiredxe2x80x94already within one period.
A circuit arrangement for monitoring of a defined amplitude threshold value of alternating voltage shaped input signals is described in the not yet published German printed patent document DE 19748633 A1of the applicant, comprising a series connection of a rectifier, a voltage divider disposed with its anode at an input signal and with its cathode feeding the voltage divider comprising at least two resistors and disposed between anode of the rectifier and mass potential and wherein the tap of the voltage divider is connected to the comparison input of the comparator such that a first binary signal is generated at the output of the comparator, furthermore with a zero passage detector, wherein the monitored input of the zero passage detector is the switch with reference signal for forming of a second binary signal, a delay stage connected to the output side of the zero passage detector for forming of a time limited third binary signal, and with at least one signal edge controlled flip-flop, wherein the comparator output is connected to a status controlled input of the flip-flop and wherein the output of the delay stage is connected to a signal edge controlled input of the flip-flop such that a status distinguishing for signal is generated at the output of the flip-flop. This switching arrangement is not suitable for a quick monitoring of a relatively high current input signal, which is realized by an input side connection switching of a parallel capacitor.
It is therefore an object of the present Invention to improve the input circuit in such a way that a turn off of devices, which deliver a relatively high input current, can be quickly captured.
Starting with an input circuit of the initially recited kind, the object is accomplished according to the present invention by the characterizing features of the independent claim, while advantageous further developments of the invention can be gathered from the dependent claims.
The voltage is lowered very quickly with respect to its amount to below the threshold value through the capacitor and thereby the input voltage is lowered very quickly to below the threshold value through the capacitor based on the discharge circuit added according to the present invention even in cases where the device to be monitored is switched off at the apex of the supply voltage, such that the switching off of the device to be monitored is rapidly captured. The quick recognizability of the switching on of the device to be monitored remains based on the low output resistance in a switched on state of the device to be monitored.
An advantageous further development of the Invention comprises that the discharge resistance is of low ohm resistance relative to the output resistance of the switched off device to be monitored. The lowering of the input voltage below the threshold value becomes recognizable at the latest after an alternating voltage period.
An advantageous further development of the invention comprises further the common and joint application of the switching element for several inputs of the same kind, in particular also upon application of only one common and joint discharge resistor in case of a not too large a number of inputs.
Advantageously, the discharge circuit comprises a series connection of rectifier, discharge resistor, and switching element; wherein the switching element is formed advantageously as a transistor.